1. Field of the Invention
The present invention relates to a tire vulcanizing machine and, more specifically, to a tire vulcanizing machine capable of vulcanizing a tire in a high quality at a relatively low cost by symmetrically expanding a bladder inserted with green tire symmetrically with respect to its horizontal center plane for shaping.
2. Description of the Prior Art
Referring to FIG. 5 showing a conventional tire vulcanizing machine of a bottom shaping type for shaping a green tire, a bottom mold 53 is held on a bottom mold holder 52 supported on a base frame 51, and a vertically movable loader 55 having a chuck 54 sets a green tire 56 in the bottom mold 53 for shaping. A central mechanism 58 is extended axially through a guide sleeve 59 attached to the central portion of the bottom mold holder 52. The central mechanism 58 comprises a lifting cylinder 60 axially movably inserted in the guide sleeve 59, a cylinder actuator 69 provided with a piston rod 61 extending through the lifting cylinder 60, an upper clamping ring 63 attached to the upper end of the piston rod 61, a lower clamping ring 65 disposed coaxially under the upper clamping ring 63 and fixed to the upper end of the lifting cylinder 60, a bladder 67, an upper bead ring 64 fastening the upper end of the bladder 67 to the upper clamping ring 63, a lower bead ring 66 fastening the lower end of the bladder 67 to the lower clamping ring 65, and a swing arm 68 for vertically moving the lifting cylinder 60 together with the piston rod 61. The piston rod 61 is moved vertically to move the upper clamping ring 63 vertically and the lifting cylinder 60 is moved vertically to move the lower clamping ring 65 vertically.
In shaping a green tire by the tire vulcanizing machine thus constructed, the lifting cylinder 60 is set at its lowermost position to place the lower clamping ring 65 at its lowermost position, and the piston rod 61 is fully extended to place the upper clamping ring 63 at its uppermost position to extend the bladder 67 taut on the bottom mold 53 coaxially with the bottom mold 53 as indicated by alternate long and two short dashes lines in FIG. 5. The loader 55 holding a green tire 56 with the chuck 54 is lowered to seat the green tire 56 on the bottom mold 53. After the green tire 56 has been seated on the lower mold 53, a pressurized fluid, such as steam or a hot gas is supplied into the bladder 67 to expand the bladder 67 and the piston rod 61 is lowered as the bladder 67 is expanded to lower the upper clamping ring 63. The bladder 67 expands gradually, conforming to the shape of the inner surface of the green tire 56. Finally the bladder 67 conforms closely to the inner surface of the green tire 56 for shaping. After the green tire 56 has perfectly been shaped, a top mold, not shown, is joined to the bottom mold 53, and then the green tire 56 is subjected to vulcanization.
Such a mode of shaping the green tire on the prior art tire vulcanizing machine is called generally a bottom shaping mode. Since the bladder 67 is expanded by supplying a pressurized fluid into the bladder 67 and the upper clamping ring 63 is lowered as the bladder 67 is expanded so that the bladder 67 is inserted in the green tire 56 in the bottom shaping mode, the bladder 67 is expanded somewhat asymmetrically with respect to the horizontal center plane thereof as the same is inserted in the green tire 56. Consequently, the green tire 56 is shaped asymmetrically with respect to its horizontal center plane and it is impossible to obtain a tire of a high quality. An invention disclosed in Japanese Patent Laid-open (Kokai) No. Sho 56-148539 employs a tire vulcanizing machine of a center shaping type. In the operation of the tire vulcanizing machine of a center shaping type, upper and lower clamping rings are located near the horizontal center plane of a bladder, and then the upper and lower clamping rings are moved away from each other, supplying a pressurized fluid into the bladder having opposite ends attached respectively to the upper and lower clamping rings. Since the bladder is expanded symmetrically with respect to the horizontal center plane thereof as the bladder is inserted in the green tire, the vulcanized tire is symmetrical with respect to the horizontal center plane thereof. Thus a tire of a high quality can be obtained.
The tire vulcanizing machine of a center shaping type that operates in the center shaping mode, however, needs a bottom mold holder provided with a cylindrical well formed in its central portion to accommodate the bladder having opposite ends attached respectively to the upper and lower clamping rings, and a lower bead ring capable of being vertically moved relative to the lower clamping ring. Thus, the tire vulcanizing machine needs a central mechanism of a complicated construction, which increases the cost of the tire vulcanizing machine.